Great efforts have hitherto been made to increase sensitivity of silver halide photosensitive materials. In the case of sensitizing dyes, it is considered that increase in their light absorption factor will render possible improvement of the transfer efficiency of light energy to a silver halide and achievement of increased spectral sensitivity.
However, the amount of adsorption of a sensitizing dye on the surface of silver halide grains has a limitation, so that it is difficult to adsorb the sensitizing dye more than its single layer saturation adsorption. In consequence, it is the current situation that the absorption factor of incident light quantum of individual silver halide grains is extremely small in the spectral sensitization range.
The following describes methods which have been proposed for the purpose of resolving these problems.
In Photographic Science and Engineering, vol. 20, no. 3, p. 97 (1976), P. G. Gilman, Jr. et al. have adsorbed a cation dye to the first layer and an anion dye was adsorbed to the second layer making use of electrostatic force.
In U.S. Pat. No. 3,622,316, G. B. Bird et al. have attained sensitization through the contribution of Forster type excitation energy transfer, by adsorbing a plurality of dyes to multiple layers of a silver halide.
In JP-A-63-138341 and JP-A-64-84244 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), Sugimoto et al. have carried out spectral sensitization by energy transfer from a luminous dye.
In Photographic Science and Engineering, vol. 27, no. 2, p. 59 (1983), R. Steiger et al. have attempted to effect spectral sensitization by energy transfer from a gelatin-substituted cyanine dye.
In JP-A-61-251842, Ikekawa et al. have carried out spectral sensitization by energy transfer from a cyclodextrin-substituted dye.
So-called connected dye which has two chromophores not separately conjugated but connected by covalent bond is disclosed, for example, in U.S. Pat. Nos. 2,393,351, 2,425,772, 2,518,732, 2,521,944, and 2,592,196, European Patent 565,083 and the like. However, these attempts were not aimed at improving light absorption factor. As a positive effort to improve light absorption factor, G. B. Bird, A. L. Borror et al. in U.S. Pat. Nos. 3,622,317 and 3,976,493 have attempted to increase sensitization through the contribution of energy transfer by increasing light absorption factor which was effected by adhering a connected type sensitizing dye molecule having a plurality of cyanine chromophores. In JP-A-64-91134, Ukai, Okazaki and Sugimoto have proposed a method in which at least one substantially non-adsorbent dye containing at least two sulfo groups and/or carboxyl groups is connected to a spectral sensitizing dye capable of being adsorbed to a silver halide.
In addition, M. R. Roberts et al. have proposed, in U.S. Pat. No. 4,950,587, spectral sensitization by a cyanine dye polymer.
Thus, though a large number of attempts have so far been made with the aim of improving light absorption factor, each of these attempts is not sufficient in improving high sensitivity effect and has problems in terms of increased specific desensitization, development inhibition and the like.
Because of the above reasons, great concern has been directed toward the development of a spectral sensitization technique which renders possible achievement of high sensitivity of a silver halide sensitive material through the improvement of its light absorption factor.